Rule Of Thumb #2

The amount of gain or loss is roughly proportional to the size of the windshift.

When the wind shifts, the amount that any boat gains or loses will depend on two factors. One of these is the magnitude of the shift. Basically, the bigger the shift, the more you will gain (or lose) on other boats.
In the example shown, a 30 degree shift (C) puts the port tack boat roughly twice as far ahead of the starboard tacker as a 15 degree shift (B).